The overwhelming majority of Iranians--at least 90 percent of the total population--are Muslims who adhere to Shia Islam. In contrast, the majority of Muslims throughout the world follow Sunni Islam. Of the several Shia sects, the Twelve Imam (see Glossary) or TweÍÍÍÍlver (ithna- ashari), is dominant in Iran most Shias in Bahrain, Iraq, and Lebanon also follow this sect. All the Shia sects originated among early Muslim dissenters in the first three centuries following the death of the Prophet Muhammad in A.D. 632 (see Islamic Conquest , ch. 1).

The principal belief of Twelvers, but not of other Shias, is that the spiritual and temporal leadership of the Muslim community passed from Muhammad to Ali and then sequentially to eleven of Ali's direct male descendants, a tenet rejected by Sunnis. Over the centuries various other theological differences have developed between Twelver Shias and Sunnis.

Data as of December 1987